Vehicle engines use various sensors to provide information that is then used to control engine operation for a variety of reasons. One example system, described by U.S. Pat. No. 6,575,148, uses a specific humidity sensor to modify the engine air-fuel ratio, as well as other engine parameters.
The inventors of the present invention have recognized a disadvantage with such an approach. In particular, such a system can produce degraded results due to interrelationships between engine misfire, humidity, and the achievable fuel economy performance. Specifically, the inventors of the present invention have recognized that the achievable lean air-fuel ratio combustion limit varies as ambient humidity varies. In other words, if a lean air-fuel ratio is optimized for low humidity (as lean as possible to maximize fuel economy in low humidity conditions), an increase in humidity may cause a change in the mixture dilution that results in engine misfire. Likewise, if the lean air-fuel ratio is set for a worst case of high humidity, thereby reducing engine misfires, this results in less vehicle fuel economy and increased emissions on low humidity days. As such, operation according to prior approaches results in either increased engine misfires, or lost vehicle fuel efficiency and increased emissions.